In ink jet printing technology, the main issues are to improve the quality and speed of printing.
Almost all the printing technologies developed today have the objective of producing high quality copies as fast as possible. In the case of ink jet technologies, one way to achieve fast printing is to multiply the number of nozzles that can eject ink drops on the head surface to print a larger number of points in parallel on the receiving support. However, the number of nozzles on the head surface is limited either because of problems related to heat dissipation in methods involving heating the ink to a high temperature, or because of problems related to dimensional instability due to the vibrations in methods using piezoelectric technologies.
One of the conventional technologies for producing ink jet heads comprises heating the ink found in a channel to a temperature, of usually from 300 to 400.degree. C., in a very short time such as a few microseconds. This leads to local vaporization of the ink that causes the expulsion as drops of the liquid part of the ink found between the vaporization zone and the surface of the ink jet head. This method requires thermal energy in the volume of the ink jet head itself, and this thermal energy must then be dissipated.
Other techniques, such as described European Patent Application 771,272, comprises the step of bringing the fluid into contact with a ring-shaped heating element located at the opening periphery of the channel linking a reservoir containing the fluid to the opening at the surface of the ink jet head. Pressure is applied to the reservoir in order to allow the ink to be carried through the channel and spread out onto the ring-shaped heating surface of the ink jet head. When the heating element of the ink jet head is raised to a temperature of about 130.degree. C., there follows a significant alteration of the surface tension of the ink drop found in contact with the heating element. This alteration of the surface tension causes a decrease in the radius of curvature of the ink drop meniscus thus enabling it to run freely through the channel and to form a drop of the size suitable for the printing required. Once formed, this drop is then ejected by a means such as an electrostatic field between the ink jet head and the print media, for instance a sheet of paper. This technique, which has the advantage of considerably lowering the temperature required to eject a unit volume of ink, is thus more suitable for manufacturing highly integrated ink jet heads. In theory, it is only necessary to heat the surface of the ink drop meniscus to obtain the alteration of its radius of curvature and thus its ejection; but in practice, it is necessary to heat the whole volume of the ink drop, and therefore a much higher energy is required to eject an ink drop. On the other hand, as the whole volume of the ink drop is heated, part of the energy supplied to the ink drop is still contained in it on ejection, and this facilitates the dissipation of this energy which thus does not remain confined in the ink jet head.